Microalgae and cyanobacteria (for short, algae) are micro-plants and require mostly simple mineral nutrients for growth and reproduction. By utilizing photon energy, such as sunlight and artificial illumination, algae convert, through photosynthesis, water and carbon dioxide into high-value organic compounds (e.g., pigments, proteins, fatty acids, carbohydrates, and secondary metabolites). Algae exhibit a growth potential an order of magnitude greater than higher plants because of their extraordinarily efficient light and nutrient utilization.
With over 40,000 identified species, algae represent a very diverse group of organisms. They naturally produce many novel, as yet largely untapped classes of bioproducts. Globally, annual sales of algae-derived products (pharmaceuticals, nutraceuticals, agrochemicals, human food, and animal feed) were estimated to be $2 billion in 2004. By taking advantage of the latest breakthroughs in molecular biology, metabolic engineering and functional genome research, algae can serve as an excellent gene-expression vehicle for production of recombinant proteins and other biologically active compounds for human and animal health and nutrition.
Due to the ability to rapidly uptake nutrients (such as carbon dioxide, nitrogen, and phosphorous) from the surrounding environment and convert them into organic compounds such as proteins stored in the cell, algae have been proposed and tested in natural and engineered systems to remove and recycle waste nutrients from wastewater and carbon dioxide-rich flue gases emitted from fossil fuel-fired power generators. The algal biomass produced as a by-product of the bioremediation process can then be used as feedstock for production of biofuels (such as biodiesel, ethanol, or methane), animal feed additives, and organic fertilizer.
Although application of algae for renewable biofuels of both liquid and gaseous forms and high-value products, and for environmental bioremediation is scientifically and environmentally sound, economic viability of algal applications is determined by the efficiency and cost-effectiveness of industrial-scale culture vessels, or so-called photobioreactors (for short, reactors), in which algal grow and proliferate.
Industrial photobioreactors currently are commonly designed as open raceways, i.e. shallow ponds (water level ca. 15 to 30 cm high) each covering an area of 1000 to 5000 m2 constructed as a loop in which the culture is circulated by a paddle-wheel (Richmond, 1986). This production mode has the advantage of being relatively simple in construction and maintenance, but it has many disadvantages which relate to the factors controlling productivity of algal grown outdoors (Richmond, 1992; Tredici et al. 1991). The overall low productivity of the open raceways is due mainly to the lack of temperature control and the long light-path, as well as poor mixing. The open raceways in which the algal culture is open to the air is also responsible for culture contamination with airborne microorganisms and dusts, often causing culture failure or crashes. The significant drawbacks of the open raceways have prompted the development of closed systems, i.e. photobioreactors made of transparent tubes or containers in which the culture is mixed by either a pump or air bubbling (Lee 1986; Chaumont 1993; Richmond 1990; Tredici 2004).
A number of tubular photobioreactors have been proposed and developed since the pioneering works of Tamiya et al. (1953) and Pirt et al. (1983). These solar receptor bioreactors are generally serpentine or helical in form, made of glass or plastic with a gas exchange vessel where CO2 and nutrients are added and O2 removed connected to the two ends of the tubing, and with recirculation of the culture between the vessel and tubing performed by a pump (Gudin and Chaumont 1983) or an air-lift (Pirt et al. 1983; Chaumont et al. 1988; Richmond et al. 1993). Because of their improvement in light path, culture temperature, and mixing, tubular photobioreactors not only increase considerably algal biomass productivity, but also enable more algal species of commercial interest to grow and proliferate under more controllable culture conditions.
On the other hand, the tubular-type photobioreactors suffer from their own inherit problems. First of all, tubular photobioreactors have a significant ‘dark zone or dark volume’ (usually consisting of 10-15% of the total culture volume) associated with a degas reservoir/tank where the exchange of excess amounts of dissolved oxygen with carbon dioxide occur. Algal cells entering the dark zone cannot perform photosynthesis, but consume, through cellular respiration, cell mass which have previously been assimilated under light. As a result, a tubular reactor will only sustain biomass yield of 85-90% of the theoretical maximum. Secondly, tubular photobioreactors have the potential to accumulate in the culture suspension high concentrations of molecular oxygen evolved from photosynthesis, which in turn inhibits photosynthesis and thus biomass production potential. Thirdly, mechanic pumps that are commonly employed by tubular photobioreactors to facilitate culture mixing and circulation within long tubes can cause serious cell damage. For example, some 15% of the damage to cells has been reported to be associated with operation of tubular-type bioreactors (Shilva et al. 1987). Gudin and Chaumont (1991) also observed that significant cell fragility occurred in a Haematococcus culture maintained in a large-scale tubular photobioreactor. Due to severe hydrodynamic stress created by various mechanical pumps, only a limited number of algal species are able to survive in a tubular bioreactor. Also, the high capital and maintenance costs associated with tubular photobioreactors have limited their applications only for production of small quantity, high-value specialty products.
During the last ten years, however, attention has focused on flat plate-type photobioreactors. This type of reactor was first described by Samson and Leduy (1985) and by Ramos de Ortega and Roux (1986), and further refined by Tredici et al. (1991, 1997) and Hu et al. (1996, 1998a,b). Flat plate-type designs offer greater advantages over the tubular-type systems: 1) no “dark zone” is associated with the flat-plate design and the reactors are illuminated in their entirety, thus boosting photosynthetic productivity; 2) aeration that facilitates culture mixing and turbulence exerts little harm to algal cells because of the minimum hydrodynamic force created by air bubbling; 3) harmful levels of oxygen are not built up in flat plate-type system because of their short reactor heights (i.e., 3 to 10 feet); 4) flat-plate reactors can be set at various orientations and/or tilted angles aimed at maximal exposure to solar energy throughout the year to further enhance photosynthetic biomass yield; and 5) compared to tubular reactors, flat-plate reactors require considerably less capital and maintenance costs.
However, application of flat plate-type reactors has encountered a major engineering obstacle, i.e., difficulty of scaling up the flat plat-type design to a commercial level. Therefore, flat plate reactors have only been used as bench-top culture devices and as small outdoor culture units for study of algal growth physiology, and have never been applied to industrial cultivation of algae.